1. Technical Field
The present invention relates to the registration of images acquired using different imaging techniques, and more specifically, to the alignment of fat saturated and non-fat saturated T1 weighted images.
2. Discussion of the Related Art
Fat saturation (also known as “fat suppression”) techniques are often used in Dynamic Contrast Enhanced Magnetic Resonance Imaging (DCE MRI). Fat saturation techniques eliminate the otherwise high intensity signal obtained from fat that makes the interpretation of abnormal enhancements difficult.
Non-fat saturated images are also useful in the interpretation of DCE MRI because in the non-fat saturated images, normal glandular tissue (e.g., breast tissue) appears dark and allows normal enhancements to be distinguished.
DCE MRI produces 3D anatomical structure information of tissues. The analysis of tissue voxel intensities produced by DCE MRI provides a noninvasive assessment of the tissue characteristics.
The T1 relaxation time is a biological parameter that is used in MRIs to distinguish between tissue types. This tissue-specific time constant for protons, is a measure of the time taken to realign with the external magnetic field. The T1 constant will indicate how quickly the spinning nuclei will emit their absorbed RF into the surrounding tissue.
As the high-energy nuclei relax and realign, they emit energy that is recorded to provide information about their environment. The realignment with the magnetic field is termed longitudinal relaxation and the time in milliseconds required for a certain percentage of the tissue nuclei to realign is termed “Time 1” or T1.
Starting from zero magnetization in the Z direction, the Z magnetization will grow after excitation from zero to a value of about 63% of its final value in a time of T1. This is the basic of T1 weighted images.
The T1 time is a contrast determining tissue parameter. For example, the slow molecular motion of fat nuclei results in a short T1 time. In contrast, the high mobility of water molecules results in a long T1 time.
The term T1 -weighted is used to indicate an image where most of the contrast between tissues is due to differences in the T1 value.
T1 fat saturated images and T1 non-fat saturated images acquired from the same patient are often misaligned because of the patient's movement (e.g., rotation) during the MRI.
For a computer aided diagnosis or detection system with access to images obtained using different imaging techniques, it is important to accurately align all structures in both images to take advantage of their complementary information.